In the field of solid state, thin film electronic devices, a variety of applications make use of materials which provide for ion transport within or between layers. A first example is in thin film batteries for charge storage, such as lithium ion and lithium metal batteries, in which lithium ions move in or through an electrolyte layer disposed between two electrode layers, an anode and a cathode. A further sub-set of applications include electrochromic (EC) devices, which in some cases also use lithium ion transport in an electrolyte layer between a reference electrode layer and a counter electrode layer to effect an optical change in device transition.
In either of such or other similar solid state thin film electronic devices, a variety of complications can be encountered. For example, a number of implementations might involve the use of a first lithium based layer, as for example, either a lithium metal electrode or a lithiated electrode or electrolyte layer, lithiated in the sense of including intercalated Li+ ions within the layer. However, the lithium in such a layer may be relatively volatile or unstable in that exposure of that layer, and thus the Li ions, to air (oxygen, O2, or water vapor, H2O) may allow for the lithium to react with oxygen, which oxidizes the lithium and thereby renders the lithium unavailable for ion transport. The more stable lithium oxide (LiO) produced by such an oxidation reaction does not provide free Li+ ions available for movement and electrochemical action.
Examples where such air exposure could occur may include manufacturing instances where either or both a lithiated electrode layer is exposed to air during a mask change, or when an overlayer is of such a nature, e.g., porous, such that air exposure thereto could allow for oxygen permeation or passage to the underlying lithiated layer. A more specific example may include use of a relatively porous lithium-based electrolyte layer, e.g., a porous lithium aluminum tetrafluoride (LiAlF4) overlayer deposited over a lithiated electrode layer, such as a lithiated nickel oxide (NiO) electrode layer or other lithiated metal oxide layer (lithiated NiO may also be used as a counterelectrode layer in EC devices, inter alia). As may be typical, a mask change may be desired after deposition of the porous electrolyte layer; however, this may thus open the lithiated electrode layer to exposure to oxygen via permeation of the oxygen through the porous electrolyte layer. A more stable, less Li+ ion mobile, LiO—NiO electrode layer may result leaving relatively fewer Li+ ions for effective conduction.
Moreover, even though it may be that a more dense, less porous overlayer may be less permeable to oxygen; the use of a rather porous electrolyte layer can have desirable attributes mechanically and electrically or electrochemically (i.e., better ion conductivity); thus, substitution with a denser, less porous layer simply to avoid air exposure may not be a preferable option. For example, a dense thin lithium-based electrolyte layer, such as lithium aluminum tetrafluoride (LiAlF4) may be used as a deposition layer over a lithiated electrode to avoid oxygen permeation; however, such a dense electrolyte may not be as conductive and/or may not be structurally sound and rather be subject to cracking and thus allow for undesirable electrical shorting therethrough.
Thus, it may be found desirable to provide a thin film solid state device which is protected from lithium ion oxidation and includes one or more of structural and electrical or electrochemical stability.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the present specification and a study of the drawings.